Fiber Bragg grating (FBG) sensors have a wide range of applications such as pressure-strain sensors, temperature sensors, micro-bending sensors and external refractive index sensors. As these optical sensors are inherently immune from electromagnetic interference and chemically inert, they are very attractive in bio-chemical applications and hazardous surroundings.
The sensing mechanism most often used in FBGs arises from the fact that the reflection wavelength for the forward propagating core mode varies linearly with temperature and strain. Since the wavelength can be measured with an accuracy of 10 pm relatively easily near 1550 nm, this represents a relative resolution of about 6 ppm. A variant of the same concept uses so-called Long Period Gratings (LPG) where coupling occurs between the forward propagating core mode and forward propagating cladding modes. In this case, the sensitivity of the resonance wavelength to perturbations can be greatly enhanced for some of the cladding modes. Furthermore, since LPGs involve cladding modes there has been great interest in using these for refractive index sensing by immersing the fibers in the medium to be measured. Special absorbing coatings can also be used to detect chemicals or liquids through the refractive index changes (or volume changes) induced in the coatings. However, the spectral response of LPG resonances is rather broad (width greater than 10 nm) making high accuracy measurements of small wavelength changes more difficult than with FBGs. Ideally, such refractive index sensors should be able to distinguish different kinds of perturbations, and insensitivity to temperature is often particularly desirable. A problem with both FBG and LPG sensors is that they are intrinsically quite sensitive to temperature, with resonance wavelengths drifting by about 10 pmfC, unless special bulky packaging is used to athermalize the device. In order to circumvent this problem in refractive index sensors, devices proposed so far have involved combination of gratings in one sensor such as two different types of fiber Bragg gratings, two fiber Bragg gratings with different cladding diameters and a long period grating (LPG) with a Bragg gratings. In such cases, the differential sensitivity of the two gratings to temperature and the desired measurand is used to discriminate between the two perturbations.
Against this background it can be clearly seen that the current sensor technology has drawbacks. It is therefore the aim of the present invention to alleviate those drawbacks.